1. Field of the Invention
The present invention relates to an electric power conversion assembly comprising switching elements for converting power by switching.
2. Description of the Related Art
FIG. 3 is a block diagram showing an electric power conversion assembly for driving an alternating-current (AC) load such as a three-phase AC motor by converting a direct-current (DC) power supply into a three-phase alternating current.
This electric power conversion assembly includes: a switching power module 1 having switching elements 2; smoothing capacitors 8 for smoothing the direct current power supply supplied to the switching elements 2; and a control circuit portion 9 for controlling the switching elements 2.
The switching power module 1 includes: switching elements 2 such as transistors, insulated-gate bipolar transistors (IGBTs) or metal-oxide semiconductor field-effect transistors (MOSFETs), etc., for converting power from a direct current to a three-phase alternating current; free-wheel diodes 3 for converting power from a three-phase current to a direct current; snubber capacitors 4 for suppressing surges generated by the switching elements 2 during switching; and a drive circuit portion 5 for driving the switching elements 2. Here, the main properties required of the snubber capacitors 4 are good frequency characteristics and film capacitors are generally used.
The smoothing capacitors 8 smooth voltage jumps, etc., by suppressing voltage fluctuations in the DC power supply 6 supplied to the switching elements 2, and since it must have a sufficiently large capacitance, aluminum-electrolyte capacitors, which easily provide large capacitance, are generally used.
The control circuit portion 9 controls the switching elements 2 by outputting control signals to the drive circuit portion 5 in the switching power module 1.
Moreover, since the drive circuit portion 5 and the control circuit portion 9 are general circuits for driving and controlling an AC load 7 such as a three-phase alternating current motor, details thereof have been omitted from the diagram.
FIG. 4 is a diagram showing the internal construction of the electric power conversion assembly in FIG. 3.
In the drawing, a module main body 30, smoothing capacitors 8, a snubber capacitor board 11 incorporating snubber capacitors 4 as a circuit, and a control circuit board 12 incorporating the control circuit portion 9 as a circuit are housed in a case 10. Generally, the module main body 30 and the smoothing capacitors 8 are electrically connected by means of a connecting plate 13 such as a copper bus bar or a copper plate using screws 14.
Furthermore, the snubber capacitor board 11 is generally disposed above the module main body 30 and is electrically connected to the DC input wiring 18 (P, N) and the AC output wiring 19 (U, V, W) using screws 14.
The module main body 30 includes: a resin portion 31 having the DC input wiring 18 (P, N), the AC output wiring 19 (U, V, W), and the drive circuit board connections 20 insertion molded therein; a base 16; an insulating substrate 17 consisting of a ceramic substrate for mounting the switching elements 2 and the free-wheel diodes 3 on the base 16; a drive circuit board 15 incorporating the drive circuit portion 5 as a circuit; and a gel filler 22 consisting of silicon gel charged between the insulating substrate 17 and the drive circuit board 15. Moreover, in some cases, the upper portion of the drive circuit board 15 is filled with epoxy resin, etc. Furthermore, the insulating substrate may be other than a ceramic substrate.
The switching elements 2 and the free-wheel diodes 3 are secured to the insulating substrate 17 which has a conductor pattern on the base 16 with an adhesive material such as solder.
The DC input wiring 18 (P, N), the AC output wiring 19 (U, V, W), and the drive circuit board connections 20 are connected to the switching elements 2 and the free-wheel diodes 3 with connecting conductors 21 such as wire bondings. Furthermore, the drive circuit board 15 and the drive circuit board connections 20 are electrically connected to each other with solder, etc.
The gel filler 22 protects the switching elements 2, the free-wheel diodes 3, and the connecting conductors 21 so that the switching elements 2 do not fail or malfunction due to moisture, dust, etc.
Furthermore, the surface of the drive circuit board 15 on the side closest to the insulating substrate 17 (the underside of the drive circuit board 15) is completely grounded and electromagnetically shielded so that the drive circuit portion 5 does not malfunction due to switching noise generated by the switching elements 2 during power conversion.
A cooling member 23 for cooling the switching elements 2 by air cooling, water cooling, oil cooling, etc., is mounted to the housing 10. Joule heat arising from the switching elements 2 passes through the insulating substrate 17 and the base 16 and is radiated externally from the cooling member 23, cooling the switching elements 2.
Moreover, details of the mounted positioning and the method of securing the control circuit board, etc., have been omitted from the diagram.
In an electric power conversion assembly of the above construction, using an electric automobile as an example, when the vehicle is started or accelerated, discharged output from the DC power supply 6, which is a battery, is converted from direct current to a three-phase alternating current to drive an AC load 7, which is a three-phase alternating current motor. Furthermore, when regenerative braking is applied to the vehicle, regenerated power from the AC load 7 is converted from three-phase alternating current to direct current and returned to the DC power supply 6.
In an electric power conversion assembly of the above construction, aluminum-electrolyte capacitors, which have a sufficiently large capacitance to smooth the DC power supply output which is to be supplied to the switching elements 2; are used for the smoothing capacitors 8, and since the internal resistance thereof is large, the generation of internal heat due to ripple voltage fluctuation differential voltage in the DC current generated by the switching elements 2 during switching increases. In order to suppress this heat generation, the smoothing capacitors 8 are cooled by the cooling member 23, and the capacitance thereof is also enlarged.
As a result, one problem has been that the surface area and volume of the smoothing capacitors 8 has been increased, increasing the overall size of the electric power conversion assembly.
Another problem has been that when aluminum-electrolyte capacitors are used, the working temperature range is narrow, and working life is shortened due to the effects of electrolyte leakage accompanying deterioration of the seals.
Furthermore, because the entire surface of the drive circuit board 15 on the side closest to the switching elements 2 (the underside) is grounded and electromagnetically shielded so that the drive circuit portion 5 does not malfunction due to radiation noise generated by the switching elements 2 within the module main body 30, a further problem has been that electronic components can only be mounted on the upper surface of the drive circuit board 15.